"Cannot execute boolean operation"
It looks like blender has trouble performing boolean operation on meshes with large ngons. There is a very easy workaround for this problem.




Select the face that will hold the window


This single polygon has 8 vertices, more than 4 vertices is considered an ngon




Triangulate it by pressing Ctrl + T, this will make many triangles out of the big ngon
(optional step) With the polygons still selected, you can reduce the number of triangles by converting tris to quads by pressing Alt + J.




Final Result